1. Field of the Invention
The present invention relates to a protection circuit for a thyristor converter housing a unit arm composed of a plurality of thyristors connected in series with each other.
2. Description of the Prior Art
High-voltage thyristor converters are employed in various applications such as direct-current power transmission systems and reactive power compensation devices. They achieve their ratings by a series or series-parallel connection of many thyristors. Thus one technical problem with a thyristor converter of this type is how to cause its respective thyristors to share voltage and current stresses equally at the time of turn-on or turn-off of the thyristors. To this end, various measures are employed. Among these important technical problems there is the problem of an over voltage due to a partial turn-off at the time of turn-off (voltage sharing among some of the series connected thyristors which are turned off). In more detail, when many thyristors are turned off, reverse voltages are required to be applied across the corresponding thyristors during long enough intervals that all of the thyristors can withstand forward voltages subsequently applied thereto. In case these reverse voltage intervals are insufficient, when a forward voltage is applied across the thyristors, some thyristors can withstand the forward voltage, but the remaining thyristors fail to perform forward recovery, i.e., recover their features to withstand the forward voltage, with the result that they maintain their conductive states. This phenomenon is a so-called partial turn-off phenomenon. When the turn-off phenomenon occurs, the arm total voltage is applied across some thyristors which have performed their forward recovery. Thus when the voltage applied across the respective thyristors is approximately equal to or higher than the rated voltage for the respective thyristors, the thyristors will be broken over and fired without any gating signal, and thus the thyristors themselves or the converter will be broken. The prior art protection is that when the reverse voltage interval (margin angle) .DELTA.t after conducting is smaller than a predetermined value (set time t.sub.s), firing instructions are given forcedly to all the thyristors (forced firing) to cause them to be fired simultaneously in order to avoid an application of over voltages across some thyristors. Whether or not the reverse voltage interval (margin angle) .DELTA.t is sufficient as compared to the predetermined value is determined in relation to the turn-off time t.sub.f of thyristor. On the other hand, since this turn-off time t.sub.f is affected by the junction temperature of the thyristor, the current reduction rate of the thyristor before the reverse voltage interval, the forward voltage increase rate of the thyristor, the value of the forward voltage across the thyristor, etc., the worst conditions, i.e., maximum value is employed as the turn-off time t.sub.f.
A protection gating instruction signal is prepared on condition that the reverse voltage interval .DELTA.t is shorter than its set time t.sub.s to deliver it to all of the thyristors, thus to forcedly fire them. In this case, since the converter operates as an inverter, when viewed from the entire system, it can be said that the converter is forcedly brought into a commutation failure by forced firing. Thus the system is adversely affected. Namely, the protection of thyristors by forced firing is not necessarily preferable for the system.
The set time t.sub.s is usually determined in relation to the turn-off time t.sub.f (approximately 400 .mu.s) under the worst conditions of the thyristor. Namely, it is determined to be approximately 600 to 800 .mu.s (an electrical angle of 10.8 to 14.4 degrees in the case of 50 Hz) on the basis of the maximum value of the turn-off time t.sub.f in consideration of the phase difference due to voltage unbalance and the set margin etc. In this respect, when the direct current and its reduction rate are not so large, i.e., when the commutation conditions are not so severe, the turn-off time t.sub.f can be 200 to 300 .mu.s.
The above prior art has the following drawbacks:
(1) Since the reverse voltage set time t.sub.s is set under the worst operating conditions, the thyristor is subjected to forced firing even when forced firing protection is not required, for example, when the junction temperature is low or current reduction rate of the thyristor is small, thus allowing the converter to be fail in commutation, resulting in the application of a disturbance to the system.
(2) Since the reverse voltage set time t.sub.s is usually large; namely, 600 to 800 .mu.s (an electrical angle of 10.8 to 14.4 degrees in the case of 50 Hz), constant control of a control advance angle .gamma. in the inverter is usually performed by setting the control advance angle at a value larger than the reverse voltage set time t.sub.s, for example, at an electrical angle of 15 to 17 degrees in the case of a 50 Hz signal in order to prevent frequent occurrence of a protection gating signal. Such a large control advance angle would increase the reactive power of the converter, with the result that reactive power larger than is required must be supplied for the system.
(3) Since the number of series connected thyristors is ordinarily determined on the basis of the surge test voltage in consideration of an inrush surge voltage (the surge test voltage=(1.5-2.0).times.the crest value of the normal operating voltage) instead of the normal operating voltage, the converter can sufficiently withstand the normal operating voltage when more than 50 to 70% ((1/2.0-1/1.5).times.100%) of the thyristors have performed their forward recovery even if a partial turn-off phenomenon occurs. Therefore, when at least more than 50% of the thyristors have performed their forward recovery, the occurrence of a protection gating signal, which would adversely affect the system, i.e., commutation failure should be avoided. In spite of this, since the prior art protection from a partial turn-off due to an insufficient margin angle is, in a sense, an anticipatory protection, protection gating signals are produced more often than they would be needed, giving rise to the serious drawback for the system that commutation failure would be caused unnecessarily many times.
The overvoltage prevention system for the partial turn-off widely used in the prior art have been referred to above. Another system is proposed by the Japanese patent publication No. 1865/77. In this system, forward voltage sense circuits are provided one for each thyristor to sense the individual forward voltages across all of the thyristors in an analog manner. The difference in voltage between two appropriate sets of forward voltage signals output by the sense circuits is extracted. Then, the voltage difference signal is compared with a reference voltage corresponding to an allowable forward thyristor voltage (withstand voltage). When the former exceeds the latter, it is judged that a partial turn-off phenomenon has occurred. On the basis of this judgement, an operating signal is produced and then is output via a pulse shaping circuit as a protection gating instruction signal.
In accordance with this protection system, when a partial commutation failure occurs, detection of this fact is conducted to effect simultaneous firing for protection. Accordingly, this allows unnecessary simultaneous firing to be reduced to a much greater extent as compared with the first-mentioned system, thus making it possible to lessen adverse influences on the system to some degree. However, this system also has the following drawbacks:
(1) Two compared forward voltage signals are considered in an attempt to select corresponding thyristors having the maximum and minimum storage carriers Q.sub.max and Q.sub.min. These Q.sub.max and Q.sub.min elements as they are, however, will not necessarily be maximum and minimum, respectively, under all of the operating conditions, giving rise to the possibility the thyristors cannot be protected. Namely, even if a partial turn-off occurs, there could occur such a circumstance that the system is unable to detect this partial turn-off.
(2) Since the difference between the two forward voltage signals (analog signals) corresponding to the forward voltage across the thyristors is compared with a reference voltage corresponding to a voltage near the withstand voltage of a thyristor, there is no time margin from the determination to the simultaneous firing, resulting in the possibility that the thyristors may not be protected. Namely, various partial turn-off states are conceivable. Depending upon the number of elements which cannot perform their forward recovery, the voltages and voltage change rates of the elements which have performed forward recovery vary to great extent. Thus it is supposed that the thyristor voltages have already exceeded their allowable values at the time when a simultaneous-firing pulse is supplied thereto.
(3) Differently from the partial turn-off, the partial turn-on is not necessarily caused by the thyristors having the maximum storage carriers, i.e., short reverse voltage interval. In other words, even among thyristors having the same storage carriers, the thyristors having short turn-off time t.sub.f are likely to cause a partial turn-on.
Even the last mentioned prior art fails to provide secure and safe protection at the time of occurrence of the partial turn-off phenomenon.